When an audio amplifier amplifies an electrical signal that the human voice is converted into using a microphone, the output of the amplifier may saturate and distort if a speaker speaks too loudly due to lack of experience of using the microphone. Even though the speaker speaks at a suitable level, the same phenomena may happen due to the wide dynamic range of the human voice. On the other hand, the dynamic range of an audio signal such as music is sometimes narrowed by amplification. An automatic dynamic range control circuit has hitherto been used to prevent such distortions, and an automatic gain control (AGC) circuit is a well-known type of automatic dynamic range control circuit.
By automatically controlling the amplification factor of an amplifier, an automatic gain control circuit keeps the amplifier output level at or below a prescribed level. The automatic gain control circuits, for example, are introduced into acoustic electronic equipment for processing audio signals, of which dynamic range is inherently restricted.
Automatic gain control circuits are also utilized in RF (radio frequency) and/or IF (intermediate frequency) circuits of radio equipment. When a transceiver receives too strong a signal, the gain of the receiver circuit is automatically adjusted so that no intermodulation distortion caused by the amplifier is generated, thereby this makes it possible to increase the dynamic range of the receiver circuit.
Automatic gain control circuits are constructed with either feedback or feedforward circuit configurations. The following reference relates to automatic gain control circuits that employ a feedback loop circuit:
(1) Jack Smith, "Modern communication circuits", McGraw-Hill, Chapter 5.4, pp.188-198 (New York, 1986)
and the following references relate to automatic gain control circuits which employ a feedforward loop circuit:
(2) Japanese Patent Application Laid-open No. 54-137946
(3) Netherlands Patent Application No. 8300468
(4) Japanese Patent Application Laid-open No. 8-51331.
Reference (1) describes typical examples of an automatic gain control circuit employing a feedback loop configuration. In principle, however, if the input signal level abruptly changes, an AGC circuit based on a feedback loop concept will exhibit a transient response characterized by both an attack and decay time. This transient response is strongly dependent on circuit parameters such as a circuit configuration and cut-off frequency of the low-pass (loop) filter inserted in order to stabilize the operation of the feedback loop circuit. An automatic gain control circuit associated with this sort of overshoot characteristic is not applicable to a level control circuit where a certain level must absolutely never be exceeded however large an input signal becomes.
As far as it goes, a feedforward loop configuration provides a solution to this sort of overshoot problem. For example, the circuit configuration disclosed in Reference (2) sets the gain of the automatic gain control circuit in accordance with the peak level of the input signal so that the output can be kept at or below a prescribed value however large the input signal is.
However, the circuit configuration disclosed in Reference (2) results in the automatic gain control operating even when the input signal level is low, in which case due to the effects of the thermal noise inherently generated by amplifiers, the signal-to-noise ratio will naturally be lower. If an automatic gain control circuit operates in response to this sort of signal, the noise components predominate in the output signal (which has been held to a prescribed level) so that changes in the input signal level result in very poor sound quality with varying noise levels.
As opposed to this, in the circuit configuration disclosed in Reference (3) the gain is fixed for low level input signals, but is controlled in a nonlinear manner when the input signal level is high. However, because the dynamic range control is nonlinearly operated in response to an input signal, a problem encountered with this control method is that distortion is produced in the output signal.
With the circuit configuration disclosed in Reference (4), when the input signal level is less than a reference signal level the gain is fixed, thereby suppressing any rise in output noise level, and when the input signal level exceeds the reference signal level, the gain is controlled, thereby keeping the output level at or below a prescribed level.
In order to switch between a fixed-gain state and a state in which gain is variably controlled in accordance with the input signal level, the circuit configuration of Reference (4) is used for controlling the gain of a voltage-controlled amplifier circuit. The transition from the fixed-gain state to output level restriction is therefore performed over a certain range of input signal levels. Another circuit configuration that has been considered is to arrange a fixed-gain amplifier and a voltage-controlled amplifier circuit in parallel, with the fixed-gain amplifier being selected when the input signal level is smaller than a reference signal level, and being switched to the voltage-controlled amplifier circuit when the input signal level exceeds the reference signal level. In this case the propagation delays of the fixed-gain amplifier and the voltage-controlled amplifier circuit would be made equal in advance so that the waveform exhibits no distortion on the time axis when the changeover is made from the fixed-gain amplifier to the voltage-controlled amplifier circuit.
However, any difference in the gain of the fixed-gain amplifier and the voltage-controlled amplifier circuit will result in discontinuity on the amplitude axis and in resulting waveform distortion.
It is an object of the present invention to overcome these problems and to provide an automatic dynamic range control circuit such that no discontinuity occurs in the output signal waveform when a fixed-gain amplifier is switched over to a voltage-controlled amplifier circuit, and vice versa, in accordance with the input signal level.